Modern wind turbines comprise a plurality of wind turbine rotor blades, typically three blades, each blade having a weight of up to 15 tons and a length of up to 55 meters or longer.
Traditionally, a blade comprises two shell parts, one defining a windward side shell part and the other one defining a leeward side shell part. Each of the shell parts are traditionally made in one piece. To reinforce such a blade, a beam- or box-shaped, tubular, and longitudinal element, i.e. a spar, can act as a reinforcing beam running lengthways, i.e. in the longitudinal direction of the blade. The spar is located in the cavity between the two wind turbine shell parts and extends substantially throughout the length of the shell cavity in order to increase the strength and stiffness of the wind turbine blade. A blade may further be reinforced by two or more spars placed lengthways side by side.
During operation of the wind turbine, each wind turbine blade is exposed to considerable loads and moments both in the longitudinal direction of the blade mainly resulting from centrifugal forces, in the flapwise direction dominated by flapwise bending moments from aerodynamic thrust loads, and in the edgewise direction mainly from edgewise gravity dominated loads acting on the blade.
As the size of wind turbines and thus wind turbine blades are still growing, the production facilities and the transport means must be increased to handle blades of the required size. This also increases the demand on logistics and increases the associated costs.
Wind turbine blades manufactured in parts for later joining are known, however with major problems on obtaining the necessary strength of the joints between the connected blade parts and for safely transfer of the loads and moments across the joint. Further, difficulties may arise in designing and making the blade joints without jeopardizing the stiffness and weight considerations on the blades.